At least 65 people were killed and hundreds injured, many of them children, when a suicide bomber blew himself up at a crowded park in the Pakistani city of Lahore where Christians were celebrating Easter Sunday (Mar 27).

Officials said the bomber had detonated the explosives packed with ball bearings near a children's playing area in the popular Gulshan-i-Iqbal park.

Witnesses said they could hear children screaming as people carried the injured in their arms, while frantic relatives searched for loved ones in nightmarish scenes at the park near the centre of the city of eight million.

Doctors described frenzied scenes at hospitals, with staff treating casualties on floors and in corridors, as officials tweeted calls for blood donations.

"It was a suicide attack. The bomber managed to enter the park and blew himself up near the kids' playing area where kids were on the swings," Lahore's top administration official Muhammad Usman told AFP.

Javed Ali, a 35-year-old resident who lives opposite the park, said the force of the blast had shattered his home's windows.

"After ten minutes I went outside. There was human flesh on the walls of our house. People were crying, I could hear ambulances. It was overcrowded because of Easter, there were a lot of Christians there. It was so crowded I told my family not to go."

Yousaf Masih, a father who was searching for his family, told AFP: "My kids came here (to the park). I was at home, I saw the news on TV, but my wife and children were here."

Witnesses said the wounded were first rushed to hospital in rickshaws and other vehicles before dozens of ambulances arrived on the scene.

Usman said the death toll had reached 65 people, with more than 50 children among the injured. A Lahore rescue official confirmed the toll and said the number of injured stood at 340.

The attack was the year's deadliest, with officials saying the toll was set to rise. There was no official confirmation of who was behind the attack late Sunday.

COWARDLY, APPALLING

Sunday's blast was condemned by Prime Minister Nawaz Sharif, who "expressed grief and sorrow over the sad demise of innocent lives," according to a statement by his office.

He was later phoned by his Indian counterpart Narendra Modi who said "the people of India stand with their Pakistani brethren in this hour of grief", according to state media.

UN Secretary General Ban Ki-moon on Sunday condemned the bombinb, calling it an "appalling" act of terrorism.

"The secretary general strongly condemns the suicide bombing today at Gulshan-i-Iqbal Park in the Pakistani city of Lahore," a UN statement said.

"The secretary general calls for the perpetrators of this appalling terrorist act to be brought swiftly to justice, consistent with human rights obligations."

The Vatican also condemned the attack, calling it "fanatical violence against Christian minorities."

The United States meanwhile labelled the incident "cowardly" and "appalling", while Pakistan's Nobel Peace Prize winner Malala Yousafzai tweeted: "Pakistan and the world must unite. Every life is precious and must be respected and protected."

US presidential contender Donald Trump tweeted "Another radical Islamist attack, this time in Pakistan ... I alone can solve."

A military spokesman described the blast as a "suicide attack", adding that intelligence agencies were chasing all leads. Lahore officials said the army had been called to the scene of the attack.

On social media Pakistanis were retweeting the call for blood donations, while Facebook activated its "Safety Check" for Lahore.

The government of Punjab province declared three days mourning.

Christians are a minority in the Muslim giant of around 200 million people, making up an estimated 1.6 percent of the population, and have long faced discrimination.

Attacks targeting children carry a special resonance in Pakistan, still scarred by its deadliest ever militant assault in which Taliban gunmen killed more than 150 people at a school in Peshawar in 2014, the majority of them students.

A military operation targeting insurgents was intensified after that attack, and in 2015 the number of people killed in militant assaults dropped to its lowest since the Pakistani Taliban were formed in 2007.

Lahore, capital of Punjab province, has been relatively peaceful in recent years. But the insurgents have demonstrated a chilling ability to continue attacks on soft targets.

In January 2016 the Pakistani Taliban launched an assault on a university in Charsadda, near Peshawar, that left 21 dead and spurred a call to arm teachers as parents spoke of fears for their children.

CHAOS IN CAPITAL

Sunday's blast came as the army was also deployed on the streets of the capital Islamabad after thousands of protesters clashed with police in chaotic scenes, throwing stones and setting part of a Metro station on fire.

The demonstrators were supporters of Islamist assassin Mumtaz Qadri, who was hanged on Feb 29 for killing a Punjab governor over his call for blasphemy reform.

Analysts called the execution a "key moment" in Pakistan's long battle against religious extremism. But it has also exposed deep religious divisions in the conservative Muslim country.

The protesters issued a list of demands calling for Qadri to be named a hero and the country's controversial blasphemy laws, which carry the death penalty, to be upheld.

A military spokesman said late Sunday that the army had been deployed to secure the Red Zone around Parliament as the protesters appeared to settle in for the night.